THE UNSEEN MANIAC in Saw taunts one of his many victims, whom he has tangled up in one of his heavy-machinery death traps: "Up to now, you've sat in the shadows, watching people live out their lives ..." Well, you don't have to study film theory to know whom the serial killer meanshe means you, you jaded, motion-picture-ticket-buying voyeur, you! The killer is nicknamed "Jigsaw" and is usually seen as a sinister Mr. Punch puppet with spirals on his cheeks. In reality, he's a lumbering figure in a silk hood and robe. At the beginning he has immured a pair of men in an abandoned industrial washroom, last visited by one and perhaps several people with diarrhea.

As if in some little-theater drama of existentialism, the two are chained up and uncertain of how they got there. Between them is a dead, bloody corpse who has just blown out his brains. "I went to bed in my shithole apartment and woke up in a shithole!" cries Adam (Leigh Whannell); across the room, Dr. Lawrence Gordon (Cary Elwes) gradually realizes that this ordeal is some kind of punishment for his own infidelity. Presently, they find a cassette tape left by the maniac, who tells the doctor that he's made hostages of Dr. Gordon's wife and child, and will kill both at 6 o'clock.

The hostagesmom is played by a skidding Monica Potterblubber for their lives in a way destined to make the sensitive viewer think, "Aw, I can't bear it, put 'em out of their misery right now." It's like what's said of Herschell Gordon Lewis' grindhouse blood-feasts: if the acting were any good, it would all be unwatchable. And Elwes's acting puts Saw into what the aviation industry euphemistically calls "an uncontrolled descent"; the film might have been more fun if he were less of a whimpering family man and a bit more defiant. As one of cinema's most incompetent homicide detectives, Danny Glover seems inert at first, searching for the film's tone. Maybe he's unwilling to admit to himself, yes, this movie really is all about the torture.

In that case, Saw is best in the moments with Shawnee Smith, who plays a junkie with an infernal device locked into her jaws. Her Amanda is the lone survivor of Jigsaw's deathtraps; she even claims it was all for the best: "He helped me" ("Mad Killer Teaches Sick Life Lessons" says a newspaper headline pinned up on the police's bulletin board). Jigsaw's thing is that people don't appreciate the preciousness of life until they're almost murdered. While director James Wan might have been laughing at the American movie business's yearning for the positive spin, he seems serious: is Jigsaw really Deepak Chopra? There is a walking-wounded spirit lurking underneath the blood and the crap. And, worse, there is a Puritanical moralism: why, these men who cheat on their wives deserve to be tortured in a windowless basement! This undertone is more dismaying than any of Saw's deadly gadgets.

Saw (R; 100 min.), directed by James Wan, written by Leigh Whannell, photographed by David Armstrong and starring Cary Elwes and Danny Glover, opens Friday valleywide.